Which of the following is correct about peer to peer computing according to our course materials and readings?

Group of answer choices

$($A$)$ Peer to peer search engine requires peer to peer overlay network with centralizeed coordination.

$($B$)$ All peer to peer computing systems are distributed client$-$server systems.

$($C$)$ Peer to peer computing systems refer to distributed systems with decentralized coordination.

$($D$)$ Crowd computing is a distributed system but not a peer to peer system.

$($E$)$ Nodes in a peer to peer crawling system are serving three roles: client, server and message router.